Transform coding techniques and other systems for compressing video motion pictures are well known. One such video coding system is defined in the CCITT recommendation H.261 (Px64kbps standard). In the prior known systems, compressed video data is transmitted via channels having a constant bit-rate. An encoder buffer is an essential element of this type of video coding system. The encoder buffer is used to translate the variable bit-rate signal supplied by an encoder to a constant bit-rate signal for transmission. The encoder buffer stores the encoded information supplied to it in a first in, first out manner, typically prior to transmission. A buffer/quantizer controller is often employed to regulate the buffer fullness by specifying the quantity of information that can be supplied to the encoder buffer by the encoder.
It is important that the encoder buffer not be allowed to "overflow" or "underflow" data. Overflow of the encoder buffer causes data to be lost. Underflow of the buffer results in inefficient use of the transmission channel bandwidth. It is also important to avoid decoder buffer overflow or underflow. Decoder buffer overflow occurs when the rate at which information is arriving from the channel is greater than the rate at which the decoder can process the information. Decoder buffer underflow occurs when the decoder buffer is empty and the decoder must display a new frame but no new frame has been decoded. This can occur if the channel bit-rate is too slow.
For fixed rate transmission channels there are known control arrangements for preventing both encoder and decoder buffer overflow and underflow. However, such known control arrangements are ineffective when an actual variable bit-rate channel or an effectively variable bit-rate channel is employed to transmit the encoded video. One such actual variable bit-rate channel system is Broadband Integrated Services Digital Networks (B-ISDN). An effectively variable bit-rate channel may arise for a particular signal when encoded signals are transmitted or stored together in a multiplexed fashion, e.g., encoded video that is multiplexed with associated audio signals and other bits, such as control bits, to form a single stored multiplexed bit-stream on a video disk. The compressed video signal data, when extracted from the multiplexed bit-stream, typically becomes available in spurts such that the video data must be considered as arriving at a variable rate. This is true even when the multiplexed signal itself is being transmitted or recalled from storage at a constant bit-rate. The transmission bit-rate of such an effectively variable bit-rate channel is the rate that bits are removed from the encoder buffer associated with that channel.
A prior solution for preventing buffer overflow and underflow in a decoder when an effectively variable bit-rate channel is employed requires the use of an additional buffer memory for interfacing to the variable bit-rate channel. Use of such an additional buffer is undesirable because of cost and other limitations it imposes on system configuration.